A tunable antenna is desirable for many applications. For example in the communications field the increasing number of global wireless standards require that either a tunable antenna be used that can be operated in various frequency bands, or the deployment of an antenna for each frequency band, which requires multiple antennas. A system with multiple antennas is more expensive than a system with a tunable antenna. Also a system with multiple antennas has practical disadvantages, such as size and where to locate the multiple antennas.
In the prior art tunable antennas have been described that use varactor diodes for frequency tuning. Varactor diodes provide a method of varying the capacitance within a circuit by the application of a control voltage. One example in the prior art is described in “Tunable coplanar patch antenna using Varactor” by B. R. Holland, R. Ramadoss, S. Pandey and P. Agrawal, Electronics Letters 16th March 2006 Vol. 42 No. 6, which describes a coplanar patch antenna that is tuned using a varactor diode 40 on one of the radiating edges, as shown in the circuit diagram of FIG. 1 and the circuit layout of FIG. 2.
Tunable phase shifters are also important components in many microwave subsystems, including radars and communication systems. In these systems, the radiation pattern or reception pattern of the antenna may be steered without any mechanical movement by shifting the phase of each individual antenna element. In the prior art various phase shifters have been presented, including varactor diodes, MEMS based varactors, PIN diode phase shifters, and barium strontium titanate (BST) varactors.
For example, in “A Low-Loss Compact Linear Varactor Based Phase-Shifter” by J. H. Qureshi, S. Kim, K. Buisman, C. Huang, M. J. Pelk, A. Akhnoukh, L. E. Larson, L. K. Nanver and L. C. N. de Vreede, 2007 IEEE Radio Frequency Integrated Circuits Symposium, a varactor based phase shifter is described. As shown in FIG. 8, Qureshi et al. describe a transmission line based true-time-delay phase shifter that can be approximated by a ladder network composed of inductors and capacitors. However, due to the difficulty of implementing variable inductors, Qureshi et al. state that most practical true-time-delay phase shifters vary the capacitance 80 using MEMS or Schottky based varactor diodes.
Varactor diodes have the disadvantage of being extremely lossy, and tuning the frequency of an antenna using varactor diodes may be only 20 percent efficient, such as in the X-band frequency range. The losses due to varactor diodes significantly reduce antenna gain, which negatively impacts the range of radar or communication systems.
Current approaches for tunable delay line phases shifters are also extremely lossy and are slow to respond. The insertion loss can exceed 2-3 dB for a 360 degree phase shift, and the response time may be as slow as 1000 nanoseconds (ns).
What is needed is a tunable frequency antenna that has high efficiency and low loss. Also needed is a tunable delay line phase shifter with low loss and fast response time. The embodiments of the present disclosure answer these and other needs.